Widget Extension UnitTest Error: "Undefined Symbol: nominal type descriptor for [...]."

I have a Widget Extension for my app with a struct MyStruct defined in my extension, with target membership MyAppWidgetExtension. I also have a UnitTesting target, in which I want to have UnitTests for my main App as well as for the WidgetExtension.

As soon as I add a file to the UnitTesting-target, which has a var/let of type MyStruct, e.g. var foo: MyStruct? (which is recognized by the compiler by @testable import MyAppWidgetExtension within this file), executing the UnitTests yields a compiler error: "Undefines symbol: nominal type descriptor for MyAppWidgetExtension.MyStruct".

Any idea what's going wrong here?

I've found a solution, which feels like a workaround, but does the job:

• Define MyStruct as well in the main app target with target membership MyApp
• In the UnitTest use @testable import MyApp in order to be able to use MyStruct from the app target

Now the compiler does not complain anymore.